Stealing another man's girlfirend, fiancee, or wife.
man1 is walking down the street and sees a man and a woman together. he likes the woman. while man2 is away, man1 puts "moves" on woman1. When man2 returns woman1 is with man1. man2 runs away and becomes captain of a spaceship.
man1 just comiitted an act of Spocking. or he spocked woman1 from man2.